Many different systems and methods, depending on application, are available for dissolving gases in liquids. Some of the main applications are for the oxygenation of outdoor water bodies, industrial uses, and the treatment of wastewater.
The aeration/oxygenation of water in outdoor settings is necessary for the remediation of local environments subjected to high oxygen demand loads. Typical devices used to oxygenate water bodies are based on entraining atmospheric air into the top surface layer through fountains or paddle-type rotating wheels; or submersion entrainment with diffusers or bubble aerators that pump air to the bottom of the water body and allow bubbles to rise to the surface. Drop structures that create waterfalls also employ the same process of entrainment. These devices are typically operated continuously and are permanently installed equipment with high capital costs.
Many devices also exist for oxygenating water in an industrial setting such as vortex entrainment technology and equipment from several different manufacturers, including Air Products (Allentown, Pa.) and Air Liquide (Paris, France), but this type of equipment is usually not appropriate for field use because of high capital costs and the inability to operate continuously in outdoor conditions at remote locations. These devices are also typically not appropriate for high particulate water because of sensitivity to clogging.
Most of the current products and techniques for oxygenation of water were developed for large-scale wastewater treatment. The required equipment is usually permanently installed, installation is time-intensive, and there are high capital and start-up costs. Because most of the products available for oxygenation are not portable, the flexibility of their applications is quite limited. Wastewater treatment plants use two basic methods to aerate wastewater: entrainment or air via diffusers or other aeration devices; and, mechanical agitation to entrain air from the atmosphere into the water.
Entrainment diffusers create air bubbles by pumping air directly into the liquid phase through diffusers or some other type of device. The most widely used diffusers are classified as either porous or fine pore (nonporous). Porous diffusers incorporate porous ceramic tubing/plate through which air passes, creating very small bubbles that aerate the water. Diffusers that are classified as nonporous use perforated piping or tubing for introducing air to water. Other types of diffusers include jet aerators, aspirators, and U-tube aerators.
Mechanical aerators usually employ impellors and may be fixed or float-mounted. They may be classified as high-speed axial-flow pump type, slow-speed vertical turbine, submerged slow-speed turbine with sparger ring, and rotating brush. In pump-type aerators, which are used primarily in aeration lagoons, oxygen is transferred as the spray passes through the air and the area where turbulence is created around the impellors. The slow-speed vertical turbine blades, used for activated sludge, aerobic digesters, and aerated lagoons, are submerged a few inches below the water surface where air entrapment occurs in the vicinity of the turbine. Compressed air is released below the turbine with the submerged slow-speed vertical turbine with sparger ring to create oxygen transfer. Finally, the rotating brush, used mainly in oxidation ditches, is a long horizontal axle with radiating steel bristles that are partly submerged. Air transfer occurs in the immediate vicinity of the bristles.
Another type of aeration treatment technique is a stabilization pond (or oxidation pond), which is a relatively shallow body of wastewater contained in an earthen basin and used for secondary treatment of settled wastewater. Oxygen is introduced through wind mixing, mechanical aerators, or photosynthetic processes. In the stabilization pond, organic matter is stabilized through the combined action of algae and other microorganisms. Algae produce oxygen while growing in the presence of sunlight. This resultant oxygen is then used by other microbes for oxidizing organic matter, which in turn results in carbon dioxide, ammonia, and phosphates as end products. These end-products are required by algae for growth, resulting in a cyclical process and stabilization because of these combined processes. Stabilization ponds are classified as aerobic, facultative (aerobic-anaerobic), and anaerobic. An aerated lagoon is similar to an aerobic pond, but it is usually deeper, and oxygen is introduced via mechanical aerators rather than photosynthetic oxygen production.
Another process that can add oxygen to water is Dissolved Air Flotation (DAF), which was initially designed for solids flotation. In DAF, an enclosed container of water and air are compressed, thereby dissolving the air into the water. The pressure on the batch is then suddenly released and the air comes out of solution. Microbubbles are formed that attach to solids and nutrients in the water and float them to the surface. DAF is usually a batch process within settling and skimming tanks. Although not usually considered a method for oxygenation, DAF may be useful as an oxygenation technique.
Air flotation involves forming bubbles by introducing the gas phase directly into the liquid through impellers or diffusers. A DAF system undergoes the following processes: The influent is mixed with coagulants to cause flocculation or the formation of larger particles. This usually includes a rapid redox shift using ferric chloride or similar reducing agents, often with acrylic acid to enhance flocculation of fine solids. A recycle stream of effluent water is pressurized and saturated with air in the saturation tank, then added to the flocculated influent. This mixture is then released into the contact zone where the microbubbles (<100-120 μm) come out of solution under the lower pressure and attach to the flocculants, forming bubble-floc agglomerates. These agglomerates then leave the contact zone and rise to the surface of the flotation tank, forming a floating sludge layer on the surface of the water. This sludge is then removed by a mechanical surface scraper.
Most dissolved air flotation systems consist of similar components and can include coagulant mixing for flocculation, a saturated recycle stream, a contact zone, and a flotation tank. Others use a pilot plant with a static mixer, flocculation chamber, flotation chamber, saturator with recycle stream, and a filter column. Early stage DAF plants usually consist of a chemical coagulation area, flocculator, flotation tank, saturator, and a recycle stream.
Methods of gas saturation in a liquid include sparging air into the water in a pressure vessel, trickling the water over a packed bed, spraying the water into an unpacked saturator, entraining air with ejectors, and injecting air into the suction pipe of the recycle pump. Typical saturators are operated between 50 to 85 psig with bubble size decreasing as saturator pressure increases. Also, the retention time will increase as bubble size decreases. It is found that smaller bubbles result in a slower rise velocity as well as a smaller contact angle between bubble and particle, therefore increasing the possibility of collision between particle and bubble.
Discussion of Patent References
U.S. Pat. No. 5,979,363 (issued to Shaar) describes an aquaculture system that involves piping a food and oxygen slurry into a pond. U.S. Pat. No. 5,911,870 (issued to Hough) proposes a device for increasing the quantity of dissolved oxygen in water and employs an electrolytic cell to generate the oxygen. U.S. Pat. No. 5,904,851 (issued to Taylor et al.) proposes a method for enriching water with oxygen that employs a turbulent mixer to entrain the gas in the liquid. U.S. Pat. No. 5,885,467 (issued to Zelenak et al.) proposes mixing a liquid with oxygen using a plurality of plates or trays over which the liquid flows gradually downward. U.S. Pat. No. 4,501,664 (issued to Heil et al.) proposes a device for treating organic wastewater with dissolved oxygen that employs several process compartments. U.S. Pat. No. 5,766,484 (issued to Petit et al.) proposes a dissolved gas flotation system for treatment of wastewater wherein the relative location of inlet and outlet structures reportedly maximizes the effect of air bubbles in separating solids from the fluid. U.S. Pat. No. 5,647,977 (issued to Arnaud) proposes a system for treating wastewater that includes aeration, mixing/flocculating, and contact media for removing suspended solids, etc. U.S. Pat. No. 5,382,358 (issued to Yeh) proposes an apparatus for separation of suspended matter in a liquid by dissolved air flotation (DAF). U.S. Pat. No. 3,932,282 (issued to Ettelt) proposes a dissolved air flotation system that includes a vertical flotation column designed with an aim of preventing bubble breakage.
An object of the present invention is to provide a simplified, low cost method and apparatus for rapidly increasing the dissolved gas levels in a liquid. A further object of the invention is to provide a method of floating particles suspended in the fluid.